1. Field of the Invention
This invention relates to an optical recording medium having plural signal recording layers each having a reflective film formed on a transparent substrate carrying crests and valleys, and a method for producing the same. More particularly, it relates to an optical recording medium in which an approximately equal signal volume can be obtained from a signal recording layer irradiated with a light beam from the transparent substrate side and from a signal recording layer irradiated with a light beam from the opposite side by setting an asymmetry of reproduced signals in the recording layer illuminated by the light beam from the transparent substrate side so as to be substantially equal to an asymmetry of the reproduced signal in the recording layer from the opposite side, and a method for producing the same.
2. Description of the Related Art
Recently, an optical recording medium having plural recording layers has been proposed for achieving high-density recording. Such recording layer may be typified by such an optical recording medium having two recording layers in which a pair of transparent substrates are provided each having crests and valleys formed on its one major surface to record signals and a reflective film is further formed thereon to form a signal recording layer. These transparent substrates are bonded to each other with the reflective films facing each other. In such optical recording medium, the reflective film of one of the transparent substrates is designed to be semi-transparent and the reflective film of the other transparent substrate is designed as a usual reflective film.
For reproducing the information of the signal recording layers of such optical recording medium, a light beam is illuminated from the side of the transparent substrate carrying the semi-transparent film. The light beam is transmitted through the first transparent substrate, signal recording surface and the semi-transparent film in this order tp reach the usual reflective film. Thus, the information of the signal recording layer of the first transparent substrate carrying the semi-transparent film as the reflective film is reproduced by the light reflected by the surface of the semi-transparent film towards the transparent substrate, deposited to follow the profile of the crests and valleys, while the information of the signal recording layer of the second transparent substrate carrying the usual reflective film as the reflective film is reproduced by the light reflected by the opposite surface of the reflective film deposited to follow the profile of the signal recording surface.
This optical recording medium has, however, a drawback that a sufficient signal volume cannot be realized in the signal recording layer having the usual reflective film as a reflective film.
For example, in known optical recording medium having a sole signal recording layer, crests and valleys are formed on a major surface 101a of a transparent substrate 101 to form a signal recording surface and a reflective film 103 is formed thereon to form a signal recording layer. For reproducing the information of the signal recording layer of the optical recording medium, a light beam L1 is illuminated by an objective lens 102 from the side of the transparent substrate 101 to detect reflection by the reflective film 103. However, the light beam L1 is not illuminated from the side of the reflective film 103. In such optical recording medium, the film thickness of the reflective film 103 larger than the film thickness which will give a pre-set reflectance suffices. From the viewpoint of productivity, the film thickness of the reflective film 103 is set to approximately 500 .ANG..
With the above-described optical recording medium having two signal recording layers, a pair of transparent substrate, each having the signal recording layer comprised of a semi-transparent film or a usual reflective film as a reflective film on the signal recording surface, are bonded together so that the reflective films face each other, and the reproducing light is illuminated from the side of the transparent substrate carrying the reflective film. That is, with the signal recording layer of the transparent substrate carrying the reflective film, a light beam L3 is illuminated by an objective lens 107 from the side of the reflective film 106, that is from the opposite side of the transparent substrate 104 so that the information is reproduced by so-called reverse-direction reproduction.
Thus, in the signal recording layer of the transparent substrate carrying the semi-transparent film, the light beam is directly illuminated on the surface of the semi-transparent film towards the transparent substrate for directly detecting the profile of the crests and valleys on the transparent substrate. However, in the signal recording layer of the transparent substrate carrying the usual reflective film, a reproducing light beam L3 is illuminated on the opposite surface of the reflective film 106 to the transparent substrate so that the profile of the signal recording surface is detected via reflective film 106. Since the reflective film is formed within the valleys on the signal recording surface, these valleys are detected as being smaller by the thickness of the reflective film. The result is that, in the signal recording layer of the transparent substrate carrying the usual reflective film, the detected signal volume is smaller than the correct signal volume, such that a sufficient signal volume cannot be realized.
Recently, an optical recording medium having three or more signal recording layers, in addition to the above-described optical recording medium having two signal recording layers, has been proposed. In this type of the optical recording medium, only one of the signal recording layers has a usual reflective film, with other signal recording layer(s) having semi-transparent film(s). The signal recording layers are layered to form a transparent substrate with the signal recording layer having the usual reflective film facing the other signal recording layers, with the reproducing light beam being illuminated from the side of the outermost signal recording layer carrying the semi-transparent film. With this type of the optical recording medium, the reproducing light beam is illuminated on the signal recording layer having the usual reflective film from the opposite side with respect to the transparent substrate, such that a sufficient signal volume again cannot be achieved.